Austenitic stainless steels

ABSTRACT

An austenitic stainless steel alloy consisting of essentially less than 0.07% carbon, less than 0.05% manganese, 0.25-0.75% silicon, 24-26% nickel, 19-21% chromium, up to 0.75% niobium remainder iron and incidental impurities, the percentage being by weight. Optionally the steel may include yttrium or gadolinium within the range 0.025-1 weight percent.

lUnlte @lnles nlenl Inventor John Michael Francis Thornlmury, England Appl. No. 696,381 Filed Jan. 9, 1968 Patented Oct. 26, 197l Assignee United Kingdom Atomic Energy Authority London, England Priority Jan. 19, 11967 Great Britain 295l/67 AUSTENIITIIC STAllNlLESS STEELS 6 Claims, 2 Drawing Figs.

US. Cl. 75/128, 75/123 lnl. Cl. ..C22c 39/20, C22c 39/26, C22c 39/54 50 llieldoiSearch 75/128, 128.6

[5 6] References Cited.

UNITED STATES PATENTS 3,148,978 9/1964 White 75/1225 3,362,813 1/1968 Ziolkowski 75/128 Primary Examiner- Hyland Bizot Assistant Examiner-Joseph E Legru Attorney-Larson, Taylor & Hinds ABSTRACT: An austenitic stainless steel alloy consisting of essentially less than 0.07% carbon, less than 0.05% manganese, 0.25-O.75% silicon, 24-26% nickel, l92l% chromium, up to 0.75% niobium remainder iron and incidental impurities, the percentage being by weight. Optionally the steel may include yttrium or gadolinium within the range 0025-] weight percent.

BACKGROUND OF THE INVENTION This invention relates to austenitic stainless steels.

U.S. Pat. specification No. 3,148,973 discloses and claims austenitic stainless steel alloys consisting of essentially less than 0.07 percent carbon, 0.5-l percent manganese, 0.25-0.75 percent silicon, 24-26 percent nickel 19-21 percent chromium, up to 0.75 percent niobium, remainder iron and incidental impurities, the percentages being by weight.

One application for such alloys has been for parts intended for employment in the core of a nuclear reactor, because the alloys proved to be resistant to oxidation in typical reactor coolants such as carbon dioxide, water and steam. Further work has been done to see whether an improvement in oxidation resistance can be produced by varying the content of some of the constituents of the alloys by small amounts.

Two aspects of the problem of oxidation resistance present themselves, one being to try to prevent or reduce occurrence of oxidation, and the other to try to prevent or reduce spalling of formed oxide, for it will be understood by those skilled in the art that the production of an oxide film which is firmly adherent to the metal inhibits any further production of oxide. A third and connected aspect of the problem is to try to avoid or reduce the production of spalled oxides which, as a result of irradiation in a nuclear reactor, have induced radioactivity which is long-lived, for if such oxides should be carried around the coolant circuit and become lodged in some region to which access is required for maintenance purposes (for example heat exchangers in which steam for the power-producing turbines is generated), increasing concentration of oxide having long-lived radioactivity could produce a hazard for personnel required to perform such maintenance.

It is therefore an object of the invention to specify alloys suitable for the fabrication of parts intended to be in contact with nuclear reactor coolant, such alloys having improved properties in regard to oxidation resistance, antispalling, and length of induced activity of spalled oxides.

SUMMARY OF THE INVENTION 1, According to the invention in alloys as claimed in U.S. Pat. specification No. 3,148,978 the limit of manganese content is reduced. A level of 0.1 percent by weight of manganese is readily achievable and by careful control of manufacture of manufacture the level of about 0.05 percent by weight ofmanganese can be attained.

Alloys of minimized manganese content in accordance with the invention have been found to have improved oxidation resistance in reactor coolants as compared with alloys as claimed in U.S. Pat. specification No. 3,148,978 having manganese contents in the range 0.5-1 percent by weight.

1 DESCRIPTION OF THE DRAWING Both FIGS. 11 and 2 of the accompanying drawings are weight gain/time curves obtained from corrosion tests in carbon dioxide at 7,500 C. and 850 C. ofa low-manganesesteel in accordance with the present invention and a standard steel having a manganese content as claimed in U.S. Pat. specification No. 3,148,978.

The low manganese steel had the following analyzed composition.

Chromium, 20.68 percent by weight Nickel, 24.95 percent by weight Niobium, 0.28 percent by weight Carbon, 0.02 percent by weight Manganese, less than 0.05 percent by weight Silicon, 0.61 percent by weight Balance iron The standard steel had the following analyzed composition.

Chromium, percent by weight Nickel, 25.6 percent by weight Niobium, 0.70 percent by weight Carbon, 0.02 percent by weight Manganese, 0.73 percent by weight Silicon, 0.60 percent by weight Balance iron.

Specimens 2 oxidation of the steels were cold-sheared from strip, degreased and finally electropolished in a sulfuric acidorthophosphoric acid-water (2: l :2) solution. Before oxidation the specimens were vacuum annealed at l,O00 C. for 2 hours. Prepared specimens were oxidized at 750 C. and 850 C. in purified carbon dioxide containing less than 2 v.p.m. of oxygen and 3 r.p.m. of water. Individual weight gain measurements (micrograms per square centimeter) were recorded for specimens of both the steels after oxidation of 0.1,1, l0, and hours at each of the reaction temperatures. Log-log plots of the data obtained are as shown in FIGS. 11 and 2.

In both FIGS. 1 and 2 it can be seen. that the low-manganese steel was subjected to a decreased weight gain as compared with the standard steel at both 750 C. and 850 C. Thus the total weight of oxide is reduced thus reducing the concentration of spalled oxide which may collect at an undesirable region of the reactor.

As a further feature of the present invention in such alloys of minimized manganese content, yttrium or gadolinium may be added within the range 0.025-l weight percent.

In U.S. Pat. specification No. 3,148,978 maximum amounts of various impurities were specified for alloys intended to be employed for fabrication of parts intended for employment in the core of a nuclear reactor. It has now been found that to further the object of reducing the radioactive hazard due to concentrations of spalled oxides, the upper limit of cobalt impurity as specified in the said patent specification is advantageously reduced to 0.005 or even 0.001 weight percent, and that tantalum is now specified as an impurity which should not be present in amounts greater than 0.025 weight percent.

The addition of yttrium or gadolinium to alloys according to the invention has been shown to be effective in reducing spalling of oxides. A preferred addition is 0.5 weight percent and although larger additions may prove beneficial, the extremely high cost of this metal precludes greater additions in practice.

In order to produce niobium stabilized alloys with tantalum impurity within the said upper limit of 0.025 weight percent, it will probably be necessary to employ pure niobium as alloying addition at the preparation stage instead of employing ferroniobium as hitherto, since the latter contains too high a proportion of tantalum. I

lclaim:

ll. Austenitic stainless steel alloys substantially free of ferrite and consisting essentially or less than 0.07 percent carbon, up to 0.1 percent manganese, 0.25-0.75 percent silicon, 24-26 percent nickel 19-21 percent chromium, up to 0.75 percent niobium, remainder iron and incidental impurities, the percentages being by weight.

2. Alloys according to claim 1 wherein the manganese content is less than 0.05 percent by weight.

3. Austenitic stainless steel alloys as claimed in claim 1 and containing in addition a member selected from the group consisting of yttrium or gadolinium in amount in the range 0.025-1 percent by weight.

4. Austenitic stainless steel alloys as claimed in claim 2 and containing in addition a member selected from the group yttrium or gadolinium in amount in the range 0.025-l percent by weight.

5. Austenitic stainless steel alloys as claimed in claim i having the following upper limits of cobalt and tantalum as incidental impurities, cobalt 0.005 percent by weight, tantalum 0.025 percent by weight.

6. Austenitic stainless steel alloys as claimed in claim 2 having the following upper limits of cobalt and tantalum as incidental impurities, cobalt 0.005 percent by weight, tantalum 0.025 percent by weight. 

2. Alloys according to claim 1 wherein the manganese content is less than 0.05 percent by weight.
 3. Austenitic stainless steel alloys as claimed in claim 1 and containing in additIon a member selected from the group consisting of yttrium or gadolinium in amount in the range 0.025-1 percent by weight.
 4. Austenitic stainless steel alloys as claimed in claim 2 and containing in addition a member selected from the group yttrium or gadolinium in amount in the range 0.025-1 percent by weight.
 5. Austenitic stainless steel alloys as claimed in claim 1 having the following upper limits of cobalt and tantalum as incidental impurities, cobalt 0.005 percent by weight, tantalum 0.025 percent by weight.
 6. Austenitic stainless steel alloys as claimed in claim 2 having the following upper limits of cobalt and tantalum as incidental impurities, cobalt 0.005 percent by weight, tantalum 0.025 percent by weight. 